DZBC in POP grids?

[bradyrx]

I've been chatting with @dcherian on how to get DZBC into the grid returned from pop_tools.get_grid. This would be useful so we could derive DZU/DZT and append it to the grid file, per https://github.com/NCAR/pop-tools/pull/44/files (which requires DZBC).

The thought is that we could then add xgcm metrics to the xgcm.Axes object that comes out of to_xgcm_grid_dataset (#40). This would help out in e.g. a tracer budget module.

I haven't seen POP hist files that have DZBC in their output. Is this just because those runs don't have partial bottom cells? (e.g. /glade/p/cesm/community/CESM-DPLE/CESM-DPLE_POPCICEhindcast)

In cases where we have no partial bottom cells, we could just set DZBC to dz evaluated at KMT. In cases where there are pbc's, where do we get DZBC?

See #27.